1. Field of the Invention
The present invention relates to a wireless access system, and more particularly, to a method of an efficient low duty mode operation for a femto base station.
2. Discussion of the Related Art
A femtocell will be described in brief.
Femto is a prefix in the metric system, denoting a factor of 10−15. Hence a femtocell or FBS refers to an ultra-small indoor Access Point (AP) for low-power home use or office use. Although the term “femtocell” is sometimes interchangeably used with “picocell”, the former is used in the sense of a more advanced cell. The FBS is a small cellular BS connected to a broadband router and functions to connect 2nd Generation (2G) and 3rd Generation (3G) voice and data to a backbone network of a mobile communication service provider via the Digital Subscriber Line (DSL).
An investigation report was released, saying that FBSs would accelerate 3G proliferation and could be a driving force behind expansion of indoor coverage. It is forecast that by 2011, there will be 102 million users of users of femtocell products and 32 million APs worldwide. According to chief analyst of ABI Research, Stuart Carlaw, “From a technological standpoint, their better in-building coverage for technologies such as WCDMA, HSDPA and EVDO is an incredibly important aspect of service delivery. From a strategic and financial standpoint, the routing of traffic through the IP network significantly enhances network quality and capacity, and reduces the OPEX that carriers expend on backhaul.”
Femtocells can expand cell coverage and increase the quality of voice service. Mobile communication service providers are expecting that subscribers may be familiar with 3G by providing data service via femtocells. The femtocells are also called FBSs or femto Base Transceiver Stations (BTSs).
In summary, femtocells offer the following benefits.
1. Cell coverage improvement
2. Infrastructure cost decrease
3. New service offering
4. Fixed Mobile Convergence (FMC) acceleration.
One or more femtocells may be grouped on a service basis or geological area basis. For example, a femtocell group that allows access to a restricted group of Mobile Stations (MSs) is called a Closed Subscriber Group (CSG). An FBS allows access only to an MS that has subscribed to the CSG by checking the CSG Identifier (ID) of the MS.
FIG. 1 illustrates an exemplary network configuration including FBSs.
FBSs are new network entities added to a legacy network. Accordingly, the use of FBSs may bring about additions or modifications to an entire network structure. An FBS may directly access the Internet and function as a BS. Therefore, the FBS can perform almost all functions of a macro BS. In addition, the FBS may relay data from the macro BS to MSs.
In FIG. 1, the network is configured by adding a Femto Network Gateway (FNG) to the legacy network. The FNG may communicate with an Access Service Network (ASN) gateway and a Connectivity Service Network (CSN). The FNG may use an Rx interface for communicating with the ASN and an Ry interface for communicating with the CSN.
An FBS may access directly a Transmission Control Protocol/Internet Protocol (TCP/IP) Internet and receive a service from the CSN via the FNG. An MS connected to the FBS may receive services from the FNG or CSN in relation to IP Multimedia Subsystem (IMS) authentication, etc.
The FBS is connected to an AP via an R1 interface. This means that the FBS may receive a downlink channel from a macro BS. The FBS may also transmit a control signal to the macro BS.
In order to save the power and avoid interference with a neighboring base station, the femto base station may enter a low duty mode. At this time, a method for synchronizing a femto base station operated at a low duty mode with a mobile station and efficiently receiving a superframe header will be required.